There are numerous applications with the need for extremely miniaturized motors. For example, in consumer products extremely small, low weight, low power consumption and inexpensive motors are typically requested. The motion range is often in the order of millimeters and with a requested accuracy of micrometers. A number of performance properties are typically requested, e.g. high speed, silent movement, low power consumption, easily controlled positioning and high positioning accuracy, which to some extent are contradictory.
In order to reach highly accurate positioning that at the same time is relatively easily controlled, different motion mechanisms are selected, such as “walking” mechanisms, stick-slip mechanisms, “inertial” positioning or stepping mechanisms. Common for most devices operating according to such principles is that they involve a high degree of static contacting between an actuator and a body to be moved. The position accuracy relies to a large extent on this static contacting. However, the speed is often relatively low, the devices often operate at sub-ultrasonic frequencies, which may make them noisy, and the power efficiency is relatively poor. A typical example of such a device is disclosed in the U.S. Pat. No. 6,798,117.
Such devices can with a proper driving give rise to extremely smooth and thereby quiet motion, and is perfectly suited for many high-demand applications. Devices of this kind are generally based on a relatively complex motion pattern of the used actuators. This requires that several different voltage signals are provided with well defined phase shifts and/or voltage curve shapes. Furthermore, the complex driving pattern also requests large electromechanically activatable volumes producing heat during operation. The deformation of the actuators during operation also induce different kinds of material stress, which eventually may lead to cracking of electrodes, terminations and/or the electromechanically active material itself.
In some applications, however, the smooth motion is not of an equally crucial importance. Instead factors like improvements in speed, durability and complexity reductions are of higher interest.